In an internal combustion engine, a metal gasket is interposed between joint surfaces of a cylinder block and a cylinder head, followed by fastening, thereby imparting a sealing function. In particular, sealing in a region surrounding a bore portion of the cylinder block is important, and insufficient sealing in this portion causes a pressure drop in a combustion chamber and overheat. Consequently, a metal gasket which is constituted such that a thin metal plate having an opening corresponding to the bore portion is held between a pair of elastic metal substrates provided with beads concentric with the bore portion to enhance the sealing function of a peripheral portion surrounding the bore portion by utilizing repulsive force of the bead generated when the whole is fastened, has been in heavy usage (for example, see patent document 1).
FIG. 8 is a partially cutaway top view showing a metal gasket described in patent document 1, and FIG. 9 is a cross sectional view taken on the line X-X of FIG. 8. As shown therein, the metal gasket is constituted by disposing a thin metal plate B having openings 1 formed corresponding to bore portions of a cylinder block and having a plane shape in which substantially three rings are connected, and a pair of elastic metal substrates A1 and A2 having openings 1 provided corresponding to the openings 1 of the thin metal plate B and annular convex beads 2 formed on peripheral portions of the openings 1, so as to hold peripheral portions of the openings 1 of the thin metal plate B between the beads 2 of the elastic metal substrates A1 and A2. Further, the elastic metal substrates A1 and A2 of the metal gasket are also provided with similar beads 3 on peripheral portions of openings 8 for piping. The reference numeral 6 in the drawings designates a locking claw for locking the thin metal plate B to the elastic metal substrates A1 and A2.    Patent Document 1: JP-A-2003-287135